This invention relates to reading systems for reading bar code information and particularly, to a system, apparatus and method for reading bar code formats which encode data in either or both of the relative widths of the bars and spaces, and the relative heights of the bars.
Bar code reading systems for reading pre-printed bar codes are well known and are used in many diverse applications, such as, for example, shipping, manufacturing, retail checkout and mail sorting applications. Bar code reading systems are generally arranged to read one or more of the industry standard bar code formats which encode the information in the relative widths of the bars and spaces, such as, for example, Code 39 Interleaved 2 of 5, Discrete 2 of 5 Code 128, Code 92, Codabar, UPC, etc. A bar code reading system captures an electronic image of the bar code and forms a digital representation of the relative widths of the bars and spaces which can be displayed and/or provided to a host computer for subsequent processing.
Bar code reading systems are also well known that read bar code formats which encode data in either or both of the relative widths of the bars and spaces (width-modulation), and/or the relative heights of the bars (height-modulations), such as, for example, Postal Numeric Encoding Technique (POSTNET). POSTNET was developed by the U.S. Postal Service to provide an optimized bar code system for encoding address information on letter mail. Referring to FIG. 1, the basic elements of the POSTNET code are tall bars 100 and short bars 105 representing binary ones and zeros, respectively. Both the tall bars and the short bars are horizontally aligned at the bottom edges. Five bars of code represent one character. Each bar represents a numeric digit and each frame begins and ends with a tall bar 110.
Bar code reading systems designed to decode height-modulated bar code formats are, generally, arranged to capture and store two digital representations of the bar code. A first digital representation of the tall bars is obtained by horizontally scanning through the top portion (along a first visual plane 115) of the bar code where the first visual plane 115 includes the tall bars and excludes the shorter bars. A second digital representation of all the bars (both short and tall) is captured and stored by horizontally scanning through the bottom portion of the bar code (second visual plane 120), where the second visual plane 120 includes both the tall and short bars. The first and second digital representations are then compared to determine which bars are tall and which are short. The horizontal scanning in the first and second visual planes 115, 120 can be achieved using first and second linear scanning devices appropriately spaced apart, such as, for example, two linear laser scanners or two linear CCD scanners. Alternatively, the first and second visual planes 115, 120 can be acquired using a single linear scanning device, where the bar code and scanner are placed in a first orientation in which the first visual plane 115 is scanned, and then moved to a second orientation where the second visual plane 120 is scanned by the single scanning device.
It is noted that, in order to accomplish a decode utilizing this scheme, the bar code reading system must either have multiple, linear scanners or the bar code must be properly oriented and accurately moved relative to a single linear scanner thus requiring mechanical manipulation of the bar code and/or scanner. It is also noted that multiple images of the bar code must be translated into digital representations and compared before a decode can occur. As a result, bar code reading systems using these known schemes can be prohibitively complex and expensive for some applications.
Two dimensional charge coupled devices (CCD) that capture a visual image of both the height and widths of the bars and space within the bar code are also well known for reading bar code formats which encode data in either or both of the relative widths of the bars and spaces, and/or relative heights of the bars. CCD bar code reading systems of this type capture a digital representation of the visual image of the entire bar code and perform a decode by analyzing the relative heights of bars and widths of bars and spaces within the digitized representation. It is noted that these CCD bar code reading systems require digital memory and processing time to accomplish the decode. Further, it is noted that, as compared with laser based bar code reading systems, CCD reading systems of this type require a large surface area to capture the visual images of the entire bar code and the cost of the charged coupled device increases substantially as the surface area of the device increases.